In recent years a fairly substantial literature has developed with respect to the electroless method of gold plating on surfaces. U.S. patents of special interest both as to the electroless gold plating method and the problems associated with this procedure include; U.S. Pat. Nos. 3,589,916 (McCormack); 3,697,296 (Bellis); 3,700,469 (Okinaka); 3,917,885 (Baker); as well as the earlier patents and articles cited therein. Relevant articles include: Rich, D. W., Proc. American Electroplating Society, 58 (1971); Y. Okinaka, Plating 57, 914 (1970); and Y. Okinaka and C. Wolowodink, Plating, 58, 1080 (1971). This body of literature is pertinent to the present invention insofar as it discloses alkali metal cyanides as the source of the gold or related metal in the electroless bath as well as the use of alkali metal borohydrides and amine boranes as reducing agents. Thus, for example, the 1970 article by Okinaka as well as his U.S. Pat. No. 3,700,469 describes an electroless gold plating bath having the following ingredients.
(1) soluble alkali metal gold complex;
(2) excess free cyanide such as potassium cyanide;
(3) An alkaline agent such as potassium hydroxide; and
(4) a borohydride or an amine borane.
The 1971 article by Okinaka et al as well as Baker's U.S. Pat. No. 3,917,885 point out the problems associated with the use of these particular plating baths, particularly when the cyanide concentrations increased. Other problems were encountered when bath replenishment was carried out and the baths became unstable when a plating rate of about 2.5 microns was approached. The need to avoid undesirable gold precipitation from the baths is also noted.
In U.S. Pat. No. 3,917,885 the problems noted above were overcome by utilizing, as the gold or related metals source, an alkali metal imide complex formed from certain special imides. In order to maintain the electroless gold plating at the desired pH of about 11 to 14, the Baker patent suggests the addition to the bath of alkali metal buffering salts such as the citrates, etc. The need to utilize special imides in preparing the gold imide complex is an obvious commercial disadvantage.